...when Science says Religion is true.
The evolution of religion: Why belief systems are literally false and metaphorically true
Given we really do not know how technologically advanced past humans were we can deduce from basic human nature that fear of the unknown or different was a big motivating factor in banding together and coming up with like mindedness. Human believed they were the center of the universe and alternative thought was blasphemy.
Scared humans always look for something no matter how weird to hold on to.
Seemed oxymoronic if you are sane and know a heaven is an imaginary place you certainly would not believe anyone is there. A contradictory statement so if the relatives are in hell you lineage will die out.
It would be oxymoronic if you took both statements literally, but the truth expressed by metaphor is never intended to be taken as literal truth.